Intracardiac ultrasound is routinely used to monitor ablation procedures in the heart chambers. For example, a commercial intracardiac ultrasound system is the AcuNav™ of Siemens Medical Solutions.
The AcuNav™ transducer acquires two-dimensional (2D) B-mode images, and the corresponding ultrasound system displays these 2D images in real-time. Nevertheless, with only a 2D slice of the heart chamber visible at a time, the usefulness and usability of the system is in some ways limited. It is desirable to have a 3D image available. Image understanding (i.e. localizing anatomical features in the images), placement of the transducer, and observation of other catheters are easier to do with a 3D image.
A B-mode transducer as herein referred to is a brightness transducer for producing a typical grey-scale image. A preferred way of acquiring 3D ultrasound images, in the absence of transducers that can directly scan volumes, is to move a B-mode transducer around while tracking its pose (position and orientation), and compound the 2D images into a 3D image according to the pose information. For this “freehand 3D ultrasound”, as it is commonly called in the literature, calibration has to be performed. Calibration determines the relative pose of a magnetic pose sensor and an ultrasound image. Various methods have been described to perform the calibration. Calibration is, for example, described in Prager, R. W., R. N. Rohling, et al. (1998). “Rapid calibration for 3-D freehand ultrasound.” Ultrasound in Medicine and Biology 24 (6): 855-869.
Methods described in the literature have, however, typically only been developed and tested for prototype work, and are only suitable for calibrating a small quantity of prototype assemblies.